The present invention relates to a technique for starting an electronically switched, brushless, multi-phase, DC motor, without rotor position sensors.
The use of brushless DC motors is increasingly popular because of the low electrical noise generated by these motors. In such motors, a permanent-magnet rotor is typically used, and switching transistors drive current through the various stator windings. Commonly three stator windings are used, connected in a star-configuration. By connecting one of the three stator windings to a current source and another to a current sink, six different phases of stator magnetization can be defined.
If position-sensing devices (such as Hall-effect or optical sensors) are used to detect the instantaneous shaft position, then electronic switching can completely substitute for the commutation functions formerly performed by brushes. However, the sensors themselves add a cost and reliability penalty. Thus, substantial work has been invested, during the past decade, in eliminating the use of position sensors in such motors.
Once the motor is operating, the rotor position can be detected from the induced voltage on the stator windings (which will vary depending on the position and velocity of the rotor). Such back electromotive forces (BEMFs) can be detected differentially, with reference to the applied voltages. By processing these signals in order to determine the actual position of the rotor, and accordingly synchronizing switching of the excitation current through the phase-windings, the motor can be efficiently commutated.
Many systems are known for processing signals representative of these induced back electromotive forces, each having intrinsic advantages and drawbacks..sup.1 FNT .sup.1 See, e.g., Ogasawara et al., "An approach to position sensorless drive for brushless DC motors," 27 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS p 928-933 (September-October 1991); Matsui et al., "Brushless DC motor control without position and speed sensors," 1990 IEEE INDUSTRY APPLICATIONS SOCIETY ANNUAL MEETING; Ogasawara et al., "An approach to position sensorless drive for brushless DC motors," 1990 IEEE INDUSTRY APPLICATIONS SOCIETY ANNUAL MEETING; Pouilloux, "Full-wave sensorless drive ICs for brushless DC motors," ELECTRONIC COMPONENTS & APPLICATIONS v 10 n 1 1990 p 2-11 (1990) (ISSN: 0141-6219); Lin et al., "Using phase-current sensing circuit as the position sensor for brushless DC motors without shaft position sensor," 15TH ANNUAL CONFERENCE OF IEEE INDUSTRIAL ELECTRONICS SOCIETY-IECON '89; Bahlmann, "Full-wave motor drive IC based on the back-EMF sensing principle," INTERNATIONAL CONFERENCE ON CONSUMER ELECTRONICS 1989; Watanabe et al., "DC-Brushless Servo System without Rotor Position and Speed Sensor," PROCEEDINGS-IECON '87 (1987 International Conference on Industrial Electronics, Control, and Instrumentation); all of which are hereby incorporated by reference.
However, all such sensorless motors face a start-up problem: because the induced electromotive forces signals are not present when the motor is at rest, the starting position of the rotor is unknown. For this reason, several start-up procedures have been developed in order to overcome this technical difficulty.
A first, known, start-up procedure consists in initially exciting a certain winding, in order to call the rotor toward an equilibrium point (null-torque position) relative to said excited phase..sup.2 After a number of oscillations of the rotor about the equilibrium point, the rotor eventually stops in such a pre-established startup position. Thereafter, by selecting this initial excitation for start-up in a desired direction with maximum torque, the motor may be started in an optimal manner. FNT .sup.2 For example, if the motor has three windings, a complete excitation sequence comprises six different excitation phases. If the motor has 36 equilibrium points per revolution, each phase has six stable equilibrium points and six points of instability per revolution.
The principal drawbacks of this start-up procedure are a possible backward rotation during the phase of alignment of the rotor with the certain, fixed, start position, and a relatively long time required by the start-up procedure.
According to another known start-up procedure, the phase-windings of the motor are excited sequentially several times at a variable frequency. By starting with a certain frequency and by increasing the frequency so as to force the rotor to follow in an open-circuit mode the excitation sequence of the phases, the rotor accelerates until it reaches a speed at which induced back electromotive force signals (BEMFs) may be detected and processed.
The drawback of this procedure is that the rotor may not properly follow the excitation sequence, and may tend to oscillate about several equilibrium positions or to rotate backward.
A third, known, start-up procedure consists in measuring the inductance and the mutual inductance of the phase-windings of the motor. From the measured values it is possible to determine the actual position of the rotor.
A drawback of this procedure is that it is based on the results of measurements which depend from the particular construction of the motor and therefore the system, which is relatively complex, must be adapted, case by case, to the type of motor.
Other known start-up procedures are based mainly on a sequential excitation under open-circuit conditions, and differ among each other in the manner the accelerating sequence and the repetition thereof are carried out.
In many applications, backward rotation at startup must be avoided. For example, in magnetic disk drives for computers, a non-negligible backward rotation may damage the reading heads.
Thus, there is still a need for a fast, start-up procedure which will prevent backward rotation, and which is practically independent of the fabrication characteristics of the motor, and which can be implemented in an integrated circuit, preferably in the form of hard-wired logic.
These objectives are achieved with the start-up system provided by the present invention, which implements a start-up procedure which comprises: exciting a predefined phase to call the rotor toward an equilibrium point relative to said excited phase, for a pre-established fraction of the time necessary to the accelerated rotor, depending on its inertia characteristics, to reach a nearest position at which the BEMF which is induced by the motion of the rotor in any one of the phase-windings of the motor undergoes a "zero-crossing" (change of sign), and, before the occurrence of such a nearest "zero-crossing" event, digitally reading the output configuration of a plurality of comparators which assess the induced BEMFs on the phase-windings of the motor, detecting thereafter the occurrence of such a first "zero-crossing" event by one of said induced BEMFs within a pre-established period of time following the instant of said interruption of said first excitation; if said "zero-crossing" event occurs, reading a second time the output configuration of said comparators and decoding, by processing said first and second readings, the position of the rotor in respect to the phase-windings of the motor, and therefore the phase to be excited next in order to accelerate the rotor in the desired sense of rotation with a maximum torque; if said "zero-crossing" event does not occur before the termination of said pre-established period of time following the instant of interruption of said first excitation current pulse, repeating the process by exciting a different phase of the motor which is functionally shifted angularly by two phases as compared to said predefined phase.
In practice, in the worst case which may exist at the start-up instant, the maximum backward rotation of the rotor which may occur is less than an angle of rotation equivalent to the angular separation between two poles (or equilibrium positions) of the motor. For example, in the case of a motor having 36 equilibrium positions, the maximum possible backward rotation which may occur in the worst of cases, will be limited to less than (360/36) 10 degrees. In practice a maximum back rotation of about 7-8 degrees is experienced. According to a preferred embodiment, the start-up procedure further comprises a preliminary step during which all the phases of the motor are sequentially excited one or several times, each phase for a fraction of said pre-established duration of said first excitation step of the predefined phase, in order to eliminate the static component of friction, without actually moving the rotor from its rest position. This preliminary sequential excitation of the phases for periods of time insufficient to move the rotor from its rest position, determines a mechanical preconditioning of the motor which will facilitate the ensuing performance of the real start-up procedure.
The disclosed innovations provide methods, circuits and systems for starting-up in a desired forward sense of rotation a multiphase, brushless, sensorless, DC motor, while limiting the extent of a possible backward rotation. First, a predetermined initial phase is excited (thereby accelerating the rotor toward an equilibrium position for that initial phase), for only a fraction of the time necessary for the accelerated rotor to travel through a nearest angular position which would determine a "zero-crossing" in the waveform of any one of the back electromotive forces (BEMFs) which are induced by the rotor on the windings of the motor. After the elapsing of this brief impulse of excitation, the sign of the BEMFs induced in the windings of the motor are digitally read thus producing a first reading. The occurrence of a first "zero-crossing" event is monitored, and, if this happens within a preset interval of time subsequent to the instant of interruption of the first excitation impulse, the optimal phase to be excited first for accelerating the motor in the desired direction is decoded through a look-up table, and the start-up process may proceed. If such a zero-crossing occurrence is not detected within said period of time, the routine is repeated by exciting a different phase, which is functionally shifted by two phase positions from the initial phase. The maximum backward rotation that may occur in the worst of cases is sensibly less than the angular distance which separates two adjacent equilibrium positions of the rotor and in practice may be of just few degrees.